The present invention relates generally to a fixture for holding workpieces, particularly turbine blades, which are not readily suited for direct gripping, and more particularly to a fixture in which the cavity or interstitial space, between the inserted part of the workpiece and the corresponding or adjacent inner walls of the fixture is filled with a casting material.
Particularly in the case of turbine blades, the problem arises that the blades must be held by already-machined portions thereof in order, for example, to perform a grinding operation on the blade tips or blade bases, or on the inner and outer circumferences of blade guide ring segments.
Direct clamping or gripping of such workpieces without damaging them is impossible with conventional tools because of the complicated geometrical forms of the workpieces, Therefore, such workpieces are cast into a so-called "casting block" which encapsulates the workpiece up to the free portions to be treated or machined, and which can be mounted on a machine tool. The block comprises casting material, such as a hardened tin-lead-antimony alloy, which surrounds the workpiece.
After the treatment or machining of the workpiece, it is necessary to free the workpiece again from the casting block. This can be done by melting the block material or mechanically comminuting the casting block.
A particularly advantageous configuration for such a casting block, and corresponding comminuting means for this block, for freeing of the workpiece after its treatment, are disclosed in commonly owned copending U.S. Pat. application Ser. No. 790,863, filed Oct. 24, 1985 now abandoned and in the corresponding German patent application P 34 39 439.7, filed Oct. 27, 1984. Such a casting block is only a single-use securing means, which is destroyed and discarded after the treatment of the workpiece.
However, it is also known to employ a re-usable carrier element, which accepts the workpiece to be secured, and in which the remaining space between the carrier element and the workpiece is filled with a casting material Such a securing means is disclosed in German Published Patent Application DE-OS 33 02 823, assigned to the assignee of the present invention. The necessary separation of the workpiece from the carrier element is accomplished there by melting away of the casting material.